The Low-Energy Spectral Index of Gamma-Ray Burst Prompt Emission from Internal Shocks

The prompt emission of most gamma-ray bursts (GRBs) typically exhibits a non-thermal Band component. The synchrotron radiation in the popular internal shock model is generally put forward to explain such a non-thermal component. However, the low-energy photon index α∼−1.5 predicted by the synchrotron radiation is inconsistent with the observed value α∼−1. Here, we investigate the evolution of a magnetic field during propagation of internal shocks within an ultrarelativistic outflow, and revisit the fast cooling of shock-accelerated electrons via synchrotron radiation for this evolutional magnetic field. We find that the magnetic field is first nearly constant and then decays as B′∝t−1, which leads to a reasonable range of the low-energy photon index, −3/2<α<−2/3. In addition, if a rising electron injection rate during a GRB is introduced, we find that α reaches −2/3 more easily. We thus fit the prompt emission spectra of GRB 080916c and GRB 080825c.

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HIGH ENERGETIC GAMMA RAY BURSTS AND THEIR SPECTRAL PROPERTIES WITHIN THE FIRESHELL MODEL

International Journal of Modern Physics Conference Series ◽

10.1142/s2010194512006599 ◽

2012 ◽

Vol 12 ◽

pp. 385-389

Author(s):

B. PATRICELLI ◽

M.G. BERNARDINI ◽

C.L. BIANCO ◽

L. CAITO ◽

G. DE BARROS ◽

...

Keyword(s):

Numerical Simulations ◽

Observational Data ◽

Spectral Properties ◽

Gamma Ray ◽

Emission Spectra ◽

Gamma Ray Bursts ◽

Low Energy ◽

Isotropic Energy ◽

Thermal Spectrum ◽

Prompt Emission

The analysis of various Gamma Ray Bursts (GRBs) characterized by an isotropic energy Eiso ≲ 1053 ergs within the fireshell model has shown how that the observed N(E) spectrum of their prompt emission can be reproduced in a satisfactory way by assuming a thermal spectrum in the comoving frame of the fireshell. Nevertheless, from the study of higher energetic bursts (Eiso ≳ 1054 ergs ) such as, for example, GRB 080319B, some discrepancies between the numerical simulations and the observational data have been observed. We investigate a different spectrum of photons in the comoving frame of the fireshell in order to better reproduce the spectral properties of GRB prompt emission within the fireshell model. We introduce a phenomenologically modified comoving thermal spectrum: a spectrum characterized by a different asymptotic low energy slope with respect to the thermal one. We test this spectrum by comparing the numerical simulations with the observed prompt emission spectra of various GRBs; we present, as an exaple, the case of GRB 080319B.

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A new fitting function for GRB MeV spectra based on the internal shock synchrotron model

Astronomy and Astrophysics ◽

10.1051/0004-6361/201937057 ◽

2020 ◽

Vol 640 ◽

pp. A91 ◽

Cited By ~ 1

Author(s):

M. Yassine ◽

F. Piron ◽

F. Daigne ◽

R. Mochkovitch ◽

F. Longo ◽

...

Keyword(s):

Physical Model ◽

Gamma Ray ◽

Spectral Component ◽

Emission Spectra ◽

High Energy ◽

Spectral Energy ◽

Parametric Function ◽

Time Resolved ◽

Internal Shock ◽

Prompt Emission

Aims. The physical origin of the gamma-ray burst (GRB) prompt emission is still a subject of debate. Internal shock models have been widely explored, owing to their ability to explain most of the high-energy properties of this emission phase. While the Band function or other phenomenological functions are commonly used to fit GRB prompt emission spectra, we propose a new parametric function that is inspired by an internal shock physical model. We use this function as a proxy of the model to compare it easily to GRB observations. Methods. We built a parametric function that represents the spectral form of the synthetic bursts provided by our internal shock synchrotron model (ISSM). We simulated the response of the Fermi instruments to the synthetic bursts and fit the obtained count spectra to validate the ISSM function. Then, we applied this function to a sample of 74 bright GRBs detected by the Fermi GBM, and we computed the width of their spectral energy distributions around their peak energy. For comparison, we also fit the phenomenological functions that are commonly used in the literature. Finally, we performed a time-resolved analysis of the broadband spectrum of GRB 090926A, which was jointly detected by the Fermi GBM and LAT. This spectrum has a complex shape and exhibits a power-law component with an exponential cutoff at high energy, which is compatible with inverse Compton emission attenuated by gamma-ray internal absorption. Results. This work proposes a new parametric function for spectral fitting that is based on a physical model. The ISSM function reproduces 81% of the spectra in the GBM bright GRB sample, versus 59% for the Band function, for the same number of parameters. It gives also relatively good fits to the GRB 090926A spectra. The width of the MeV spectral component that is obtained from the fits of the ISSM function is slightly larger than the width from the Band fits, but it is smaller when observed over a wider energy range. Moreover, all of the 74 analyzed spectra are found to be significantly wider than the synthetic synchrotron spectra. We discuss possible solutions to reconcile the observations with the internal shock synchrotron model, such as an improved modeling of the shock microphysics or more accurate spectral measurements at MeV energies.

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Investigation of Similarity in the Spectra between Short- and Long-Duration Gamma-ray Bursts

Galaxies ◽

10.3390/galaxies6040106 ◽

2018 ◽

Vol 6 (4) ◽

pp. 106

Author(s):

Takanori Sakamoto ◽

Yuuki Yoshida ◽

Motoko Serino

Keyword(s):

Spectral Properties ◽

Gamma Ray ◽

Gamma Ray Bursts ◽

Synchrotron Emission ◽

Energy Photon ◽

Long Duration ◽

Photon Index ◽

Monitor Data ◽

Short Grbs ◽

Prompt Emission

We investigated the spectral properties of the prompt emission for short- and long-duration gamma-ray bursts (GRBs) using the Fermi Gamma-ray Burst Monitor data. In particular, we focused on comparing the spectral properties of short GRBs and the initial 2 s of long GRBs, motivated by the previous study of Ghirlanda et al. (2009). We confirmed the similarity in the low energy photon index α between short GRBs and the initial 2 s of long GRBs. Since about a quarter of our spectra of both short GRBs and the initial 2 s of long GRBs show α to be shallower than - 2 / 3 , it is difficult to understand in the context standard synchrotron emission.

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Prompt optical emission as a signature of synchrotron radiation in gamma-ray bursts

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935766 ◽

2019 ◽

Vol 628 ◽

pp. A59 ◽

Cited By ~ 16

Author(s):

G. Oganesyan ◽

L. Nava ◽

G. Ghirlanda ◽

A. Melandri ◽

A. Celotti

Keyword(s):

Synchrotron Radiation ◽

Parameter Space ◽

Gamma Ray ◽

Gamma Ray Bursts ◽

Spectral Shape ◽

Low Energy ◽

Optical Observations ◽

X Rays ◽

Thermal Component ◽

Prompt Emission

Information on the spectral shape of prompt emission in gamma-ray bursts (GRB) is mostly available only at energies ≳10 keV, where the main instruments for GRB detection are sensitive. The origin of this emission is still very uncertain because of the apparent inconsistency with synchrotron radiation, which is the most obvious candidate, and the resulting need for considering less straightforward scenarios. The inclusion of data down to soft X-rays (∼0.5 keV), which are available only in a small fraction of GRBs, has firmly established the common presence of a spectral break in the low-energy part of prompt spectra, and even more importantly, the consistency of the overall spectral shape with synchrotron radiation in the moderately fast-cooling regime, the low-energy break being identified with the cooling frequency. In this work we further extend the range of investigation down to the optical band. In particular, we test the synchrotron interpretation by directly fitting a theoretically derived synchrotron spectrum and making use of optical to gamma-ray data. Secondly, we test an alternative model that considers the presence of a black-body component at ∼keV energies, in addition to a non-thermal component that is responsible for the emission at the spectral peak (100 keV–1 MeV). We find that synchrotron radiation provides a good description of the broadband data, while models composed of a thermal and a non-thermal component require the introduction of a low-energy break in the non-thermal component in order to be consistent with optical observations. Motivated by the good quality of the synchrotron fits, we explore the physical parameter space of the emitting region. In a basic prompt emission scenario we find quite contrived solutions for the magnetic field strength (5 G < B′< 40 G) and for the location of the region where the radiation is produced (Rγ > 1016 cm). We discuss which assumptions of the basic model would need to be relaxed in order to achieve a more natural parameter space.

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Detection of Low-energy Breaks in Gamma-Ray Burst Prompt Emission Spectra

The Astrophysical Journal ◽

10.3847/1538-4357/aa831e ◽

2017 ◽

Vol 846 (2) ◽

pp. 137 ◽

Cited By ~ 24

Author(s):

Gor Oganesyan ◽

Lara Nava ◽

Giancarlo Ghirlanda ◽

Annalisa Celotti

Keyword(s):

Gamma Ray ◽

Emission Spectra ◽

Low Energy ◽

Gamma Ray Burst ◽

Prompt Emission

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Evidence of two spectral breaks in the prompt emission of gamma-ray bursts

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834987 ◽

2019 ◽

Vol 625 ◽

pp. A60 ◽

Cited By ~ 11

Author(s):

M. E. Ravasio ◽

G. Ghirlanda ◽

L. Nava ◽

G. Ghisellini

Keyword(s):

Gamma Ray ◽

Gamma Ray Bursts ◽

Low Energy ◽

Emission Region ◽

Peak Energy ◽

Standard Scenario ◽

Low Magnetic Field ◽

Short Grbs ◽

Prompt Emission ◽

Good Agreement

The long-lasting tension between the observed spectra of gamma-ray bursts (GRBs) and the predicted synchrotron emission spectrum might be solved if electrons do not completely cool. Evidence of incomplete cooling was recently found in Swift GRBs with prompt observations down to 0.1 keV, and in one bright Fermi burst, GRB 160625B. Here we systematically search for evidence of incomplete cooling in the spectra of the ten brightest short and long GRBs observed by Fermi. We find that in eight out of ten long GRBs there is compelling evidence of a low-energy break (below the peak energy) and good agreement with the photon indices of the synchrotron spectrum (respectively −2/3 and −3/2 below the break and between the break and the peak energy). Interestingly, none of the ten short GRBs analysed shows a break, but the low-energy spectral slope is consistent with −2/3. In a standard scenario, these results imply a very low magnetic field in the emission region (B′∼10 G in the comoving frame), at odd with expectations.

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Gamma-ray bursts prompt emission spectrum: an analysis of a photosphere model

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2006.1986 ◽

2007 ◽

Vol 365 (1854) ◽

pp. 1171-1175 ◽

Cited By ~ 4

Author(s):

Asaf Pe'er ◽

Peter Mészáros ◽

Martin J Rees

Keyword(s):

Gamma Ray ◽

Gamma Ray Bursts ◽

Dissipation Process ◽

Internal Shock ◽

Peak Energy ◽

Wide Range ◽

Low Energies ◽

Steep Slopes ◽

The Magnetic Field ◽

Prompt Emission

A thermal radiative component is likely to accompany the first stages of the prompt emission of gamma-ray bursts (GRBs) and X-ray flashes. We analyse the effect of such a component on the observable spectrum, assuming that the observable effects are due to a dissipation process occurring below or near the thermal photosphere. For comparable energy densities in the thermal and leptonic components, the dominant emission mechanism is Compton scattering. This leads to a nearly flat energy spectrum ( νF ν ∝ ν 0 ) above the thermal peak at approximately 10–100 keV and below 10–100 MeV, for a wide range of optical depths 0.03≲ τ ≲100, regardless of the details of the dissipation mechanism or the strength of the magnetic field. For higher values of the optical depth, a Wien peak is formed at 100 keV to 1 MeV. In particular, these results are applicable to the internal shock model of GRBs, as well as to slow dissipation models, e.g. as might be expected from reconnection, if the dissipation occurs at a sub-photospheric radii. We conclude that dissipation near the thermal photosphere can naturally explain (i) clustering of the peak energy at sub-MeV energies at early times, (ii) steep slopes observed at low energies, and (iii) a flat spectrum above 10 keV at late times. Our model thus provides an alternative scenario to the optically thin synchrotron–synchrotron self-Compton model.

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